1. Field of the Invention
This invention relates to a lithographic printing plate and more particularly, it is concerned with a lithographic printing plate precursor of direct imaging type, suitable for a printing plate precursor for office work.
2. Description of the Prior Art
Lately, a lithographic printing plate of direct imaging type, having an image receptive layer on a base, has widely been used as a printing plate precursor for office work. For carrying out plate making, i.e. imaging on such a printing plate, there have generally been employed a method comprising drawing an image with an oily ink by hand on an image receptive layer, or a method comprising printing it by means of a typewriter, ink jet system or transfer type thermosensible system. Furthermore, there has lately been proposed a method comprising subjecting a light-sensitive material to processings of statically charging, exposing and developing using an ordinary electrophotographic copying machine (plain paper copy machine, PPC), thus forming a toner image on the light-sensitive material and then transferring and fixing the toner image to an image receptive layer. In any case, a printing plate precursor after plate making is subjected to a surface treatment with an oil-desensitizing solution (so-called etching solution) to render a non-image area oil-desensitized and then applied to lithographic printing as a printing plate.
A lithographic printing plate of direct imaging type of the prior art generally comprises a base such as paper, a back layer provided on one side of the base and a surface layer, i.e. image receptive layer provided on the other side of the base through an interlayer. The back layer or interlayer is composed of a water-soluble resin such as PVA and starch, water-dispersible resin such as synthetic resin emulsions and pigment. The image receptive layer as a surface layer is composed of a pigment, water-soluble resin and water proofing agent.
A typical example of the lithographic printing plate precursor of direct imaging type is described in U.S. Pat. No. 2,532,865 in which the image receptive layer is composed of, as predominant components, a water-soluble resin binder such as PVA, an inorganic pigment such as silica or calcium carbonate and a water-proofing agent such as initial condensate of melamine-formaldehyde resin.
In the thus resulting printing plate of the prior art, however, there arises a problem that when the hydrophobic property is enhanced by increasing the amount of a waterproofing agent or by using a hydrophobic resin so as to improve the printing durability, the printing durability is improved, but the hydrophilic property is deteriorated to cause printing stains, and when the hydrophilic property is improved, the waterproofing property is deteriorated to lower the printing durability. At high temperatures, for example, 30.degree. C. or higher, in particular, the surface layer (image receptive layer) is dissolved in dampening water used for offset printing, thus resulting in lowering of the printing durability and occurrence of printing stains. This is an important disadvantage.
In the lithographic printing plate, moreover, drawing or imaging is carried out using an oily ink as an image area on the image receptive layer, and unless the adhesiveness of this receptive layer and oily ink is good, the oily ink on the image area is separated during printing, thus resulting in lowering of the printing durability, even if the hydrophilic property of the non-image area is sufficient and the printing stains as described above do not occur.